This application is a national stage application of International Application No. PCT/GB99/01990, filed on Jun. 24, 1999.
This invention relates to filter materials for filtering fluids and methods for the production thereof, with particular, but by no means exclusive, reference to filters for air filtration.
Air filter materials having permanent electrostatic charge are known. Such materials filter particles very efficiently due to electrostatic attractive forces between the material and the particles to be filtered. Applications include air filtration in dust helmets, respirators suction cleaners and air conditioning systems.
Filter materials desirably possess a number of advantageous properties. Such materials should filter particles efficiently while providing low resistance to the flow of fluid through the filter material. In the case of air filtration, this means a low air pressure drop across the filter material. Furthermore, it is important that the filtering performance of the filtering material is maintained, or, at least, that deterioration in performance is minimised, for as long a period of time as possible. In this way, the working lifetime of the filter is extended. Furtherstill, it is important that, once the working lifetime of the filter is over, the filter can be disposed of safely.
European Patent EP 0 246 811 describes air filter material which exhibit good filtration efficiency and low resistance to air flow due to the open structure of charged fibres employed. The material comprises a blend of polyolefine fibres and halogen substituted polyolefine fibres. U.S. Pat. No. 5,470,485 describes air filter material comprising a blend of bi-component polypropylene polyethylene fibres of the core/jacket or side-by-side type and halogen free polyacrionitrile fibres. The use of the bi-component fibres is an essential element of U.S. Pat. No. 5,470,485 so that stiff, mouldable non-woven mats of filter material can be produced. It should be noted that in each case the filter material is overall electrically neutral, fibres in the materials of different type carrying charges of opposed polarities.
The present invention provides high quality filter material which exhibits some or all of the advantageous properties described above.
According to a first aspect of the invention there is provided an electrostatically charged filter material comprising a blend of polypropylene fibres and: i) halogen free acrylic fibres or ii) polyvinyl chloride (PVC) fibres or iii) a mixture of halogen free acrylic fibres and PVC fibres.
Such filter materials exhibits excellent filtering efficiency and low resistance to the flow of fluids through the material.
Preferably, the halogen free acrylic fibres are produced by dry spinning. In a dry spinning process, the acrylic polymer is formed into fibres by extrusion into an air stream. It has been found that the filter material comprising such fibres exhibit improved filtration efficiency and better long term charge stability.
The filter material may consist of a blend of polypropylene fibres and halogen free acrylic fibres. In contrast to the filter material of EP 0 246 811, such material may be safely disposed of by incineration, since dioxin compounds are not produced.
The filter material may consist of a blend of polypropylene fibres and PVC fibres. Such material exhibits excellent long term charge stability, leading to excellent filter performance e over an extended time period.
The filter material may consist of a blend of polypropylene fibres halogen free acrylic fibres and PVC fibres. The presence of the PVC fibres in the blend provides enhanced performance over an extended time period.
The halogen free acrylic fibres may comprise at least 40%, preferably at least 50%, more preferably at least 60%, most preferably at least 85% acrylonitrile.
According to a second aspect of the invention there is provided a method for producing an electrostatically charged filter material comprising the step of blending together polypropylene fibres and: i) halogen free acrylic fibres or ii) PVC fibres or iii) a mixture of halogen free acrylic fibres an d PVC fibres.
The performance of the filter material is considerably affected by the present of lubricants and anti-static agents (collectively known as a xe2x80x9cspin finishxe2x80x9d) on the surface of the fibres. In practise, polymer fibres produced commercially have spin finishes, and thus it is necessary to substantially remove lubricants and anti-static agents from the fibres before, during or a after the blending of said fibres.
The method may comprise the step of carding the blend of fibres so as to form a non-woven fabric. The non-woven fabric may be needled.
The halogen free acrylic fibres may be produced by dry spinning.
Polypropylene fibres may be blended with halogen free acrylic fibres but not with PVC fibres.
Polypropylene fibres may be blended PVC fibres but not with halogen free acrylic fibres.
Polypropylene fibres may be blended with halogen free acrylic fibres and PVC fibres.